Recently, with the progress of scaling of semiconductor devices, processing conditions in a processing technology of semiconductor devices are diversified and a processing accuracy is desired to be improved for each processing condition. Specially, in an etching technology of using a plasma processing apparatus such as a RIE (Reactive Ion Etching) apparatus, there are increasing cases where a multilayer film is collectively processed by continuous processing. In collective processing of a multilayer film, processing conditions, such as appropriate gas flow rate, pressure, temperature, and power, are sequentially changed for each layer while continuing plasma discharge. Accordingly, it is desired to accurately control pressure of a processing gas in a processing chamber when processing a semiconductor substrate for each processing condition.